The prevalence of cardiovascular disease (CVD; $31 billion annually in medical costs) is higher in males compared to females until menopause when CVD in females surpass that of males. Impaired vascular function at rest and during exercise results from decrements in neural, humoral, and muscular activity leading to central (?cardiac output) and peripheral (blood flow (BF) maldistribution) derangements of O2 delivery and/or exercise intolerance. However, the pathways active in enhancing skeletal muscle BF distribution during exercise in healthy individuals, their decline with CVD, and whether there are male-female differences are not known. During muscle contractions, O2 delivery (QO2) is enhanced such that O2 delivery-utilization (VO2) matching (QO2:VO2 ratio) maintains capillary-myocyte driving pressure of O2 (PO2) for contractile function and exercise tolerance. Vascular adenosine triphosphate-sensitive potassium (KATP) channels support BF in health and in heart failure, and females may exhibit a greater supporting KATP channel function. Preliminary data supports both sex and muscle fiber-type differences in interstitial PO2 (PO2is) following nitric oxide synthase blockade, and decreased maximal exercise tolerance (VO2max) following KATP channel inhibition. During exercise, BF increases in a fiber-type dependent manner with oxidative muscle tissue displaying a greater reliance on KATP channels. However, the contribution of KATP channels to support PO2is during the transition from rest to exercise, and their role in establishing exercise tolerance, has not been determined. Therefore, since sex differences have not been investigated simultaneously in whole-body and peripheral KATP channel function our global hypotheses are that KATP channels contribute significantly more in female rats compared to males to support exercise tolerance (muscle BF, VO2max, critical speed (CS)) and PO2is. As such, pharmacologic KATP channel inhibition will have a greater impact in females incurring greater muscle BF and oxygenation impairment and reducing VO2max and exercise capacity. However, estrogen is not anticipated to impact KATP channel function, therefore ovariectomized females will display no difference in exercise tolerance (BF, VO2max, CS) and PO2is when compared to females with intact ovaries. We will use a novel phosphorescence quenching technique developed in our laboratory to measure the kinetic response of slow- oxidative (SO) and fast-twitch oxidative glycolytic (FOG) muscle PO2 in the space nearest contracting myocytes (PO2is) during the rest-contraction transition before and after KATP channel inhibition. These PO2is profiles will be expressed in relation to exercise tolerance (VO2max, CS) during treadmill running, with/without KATP inhibition. Understanding KATP channel contribution to exercise tolerance, and potential sex differences, is crucial in CVD where increased exercise tolerance and VO2max reduce morbidity and mortality. Any pharmacological inhibition of vascular KATP channels (i.e. glibenclamide) would be detrimental to patients especially females. The training plan incorporates the elements essential for the applicant to develop an independent scientific research career.